1. Field of the Invention
The present invention relates to a detector that can detect X-ray irradiation. More particularly, the present invention relates to a detector array that can one- or two-dimensionally measure the X-ray irradiation dose.
2. Description of the Related Art
X-ray techniques are widely used in medical examination and industrial inspection, in which a human body or a substance is irradiated with X-rays to acquire information about the inside of the human body or the substance from the absorption contrast of penetrating X-rays. Although photographic plates and imaging plates were widely used in the past to obtain transmission X-ray images, they have recently been replaced by two-dimensional X-ray detectors, in which the penetrating X-ray intensity is converted into electronic information. X-ray detectors are classified into an indirect conversion type and a direct conversion type. In the X-ray detectors of the indirect conversion type, X-rays are converted into visible light and subsequently into electronic information. In the X-ray detectors of the direct conversion type, X-rays are directly converted into electronic information. Although the present mainstream X-ray detectors are of the indirect conversion type, there is a demand for X-ray detectors of the direct conversion type that can produce high-quality images.
Improvement in the performance of X-ray detectors of the direct conversion type requires improvement in the sensitivity of an X-ray photoelectric conversion layer, which converts X-rays into electrons. X-ray photoelectric conversion layers in X-ray detectors of the direct conversion type are formed of amorphous selenium or polycrystalline Cadmium Telluride (CdTe). X-ray photoelectric conversion layers having higher sensitivity to X-rays are being developed to improve the performance of X-ray detectors. More specifically, these photoelectric conversion layers can be formed of bismuth oxide, CdTe single crystals, or lead iodide or thallium iodide, as disclosed in Japanese Patent Application Laid-Open No. 2006-49773.
However, conventional techniques for manufacturing X-ray detectors are seen to have several limitations as follows:
First: A process of forming an X-ray photoelectric conversion layer takes a long period of time. In order to improve the efficiency of absorbing X-rays, an X-ray photoelectric conversion layer must have a thickness in the range of several hundreds of microns to approximately 1 mm. The formation of such a thick film by a deposition method, such as vapor deposition, requires a long period of time and possibly increases manufacturing costs. The formation of an X-ray photoelectric conversion layer by a sol-gel method requires high temperature, often resulting in the deterioration and degradation of other components and lowering the yield.
Second: Selenium or iodine in known photoelectric conversion layers often reacts with another component of the detector, such as an electrode, causing the corrosion and deterioration of the component. This also lowers the yield.
Third: Use of single crystals, such as CdTe single crystals, in the X-ray photoelectric conversion layer entails increased costs due to their very high prices.
Fourth: Use of polycrystals, such as CdTe polycrystals, to solve the third problem results in a low yield of another component, such as a thin-film transistor (hereinafter also referred to as a “TFT”), directly formed on the polycrystals because, unlike amorphous films or single crystals, the polycrystals have a rough surface. Thus, in order to improve the yield, two substrates on which a TFT and an X-ray photoelectric conversion layer are formed separately are bonded together. However, the bonding process can also increase manufacturing costs.
Fifth: The formation of an X-ray photoelectric conversion layer having a thickness in the range of several hundreds of microns to 1 mm as described above requires the application of a high voltage (in the order of several thousands of volts) to the X-ray photoelectric conversion layer.
Sixth: Conventional X-ray detectors of the direct conversion type cannot acquire information about X-rays having different wavelengths. In other words, conventional X-ray detectors of the direct conversion type only acquire information about an X-ray having a single wavelength.